It is becoming increasingly desirable to provide a subsurface well safety valve for controlling the fluid flow through a well conduit in which the safety valve is operated from the surface by fluid pressure in which the valve does not contain elastomer seals, in which the valve can withstand pressures such as up to 30,000 psi working pressure, in which the valve can withstand high temperatures, and in which the valve can be operated at great depths. However, all of this is required to be accomplished with an outside diameter that is dictated by the size of the well casing and an inside diameter equivalent to the well tubing string.
One type of safety valve which is useful is the rod piston type safety valve disclosed in my above-mentioned copending patent application. One feature of the present invention is the provision of an improved rod type piston safety valve.
Another type safety valve is the concentric hydraulic chamber type valve such as shown in U.S. Pat. No. 3,799,258, which provides two seals of different diameters to form a hydraulic chamber. These types of safety valves have limitations for several reasons. First, the seals are positive pressure containing seals, such as elastomers or plastics, which must be placed about a wall of a tube thick enough to withstand 1.5 times the rated working pressure of the safety valve. This chamber must be housed by a threaded joint for assembly that must also withstand these pressures and therefore any such valve results in a larger than desired O.D. or smaller I.D. Secondly, any such positive pressure seal used under high differential pressures create tremendous amounts of friction on the hydraulic operating mechanism of the safety valve which must be overcome by the valve closing forces. This result in reducing the closing force thereby limiting the depth at which the valve can be utilized. Thirdly, it is useless to attempt to combat high temperatures, such as steam injections at 800.degree. F., using elastomer or plastic seals. Fourthly, in concentric designs it is generally difficult to machine a concentric hydraulic cylinder and piston to a small enough area to reduce the hydrostatic head pressure acting on the piston for allowing the safety valve to be set at greater depths. The present invention is also directed to various improvements in a concentric fluid chamber-type safety valve.